docs: def concat(cls, xs):
          """mconcat :: [m] -> m
  
          Fold a list using the monoid. For most types, the default
          definition for mconcat will be used, but the function is
          included in the class definition so that an optimized version
          can be provided for specific types.
          """
  
          def reducer(a, b):
              return a.append(b)
  
          return reduce(reducer, xs, cls.empty())